In nuclear power plants, the fuel assemblies not being used during the operation of the core of the reactors are stored in means constituted by metal cells regularly distributed in the storage pools. At present, the spacings between the cells are relatively large, because the water between the cells principally acts as a neutrophage moderator to prevent starting of a nuclear fission reaction.
However, increasing efforts are being made to increase the capacity of pools by reducing the spacings between the cells, particularly in order to permit greater flexibility with respect to reprocessing or decay delays. In other words, pools are too small to permit the storage of whole fuel assemblies irradiated during many cycles in the core of the nuclear reactor in case of postponement due to the unavailability of the reprocessing plant or of the final dry storage site. Such space reduction leads to a reduction in the thickness of the water gaps separating the walls of two adjacent cells. It is consequently necessary to compensate the moderator loss due to this reduction in the thickness in the water gap. The solution most frequently used consists of covering the walls of the cells with a neutrophage product. The most frequently used products are boron and its derivatives (boron aluminum, boron graphite, boron carbide, etc.). A first solution consists of using some of these by projecting fine particles directly on to the outer walls of the cell, and the necessary adhesion requirements may necessitate plasma projection spraying with the use of chrome and nickel as the binder. A second known solution consists of eliminating the water gap between the cells and placing the neutrophage product in a tight recess constituted by peripheral welding of contiguous walls of adjacent cells. A third solution, disclosed for example in U.S. Pat. No. 4,006,362, consists of using panels of neutrophage products, i.e., boron aluminum known by the trademark BORAL, fitted or joined to each wall of the cells in such manner that each panel ensures a like continuous layer of neutrophage protection.
The first solution is difficult to carry out and encounters major problems during manufacture of the cells. The second known solution without the water gap between the cells excludes the use of certain economically and technically suitable neutrophage materials and precludes easy dismountability of each cell located in the storage pool. The third solution fails to avoid neutron escapes through the angles of the cells because neutrophage materials do not ensure perfect continuous contact between the vertical edges of two adjacent panels in the corners of the cell. Moreover, the weight of individual modular panels presents many problems during final assembly of the cell, and the moderator quality imposes the use of thick panels.